This invention relates generally to ground fault current sensing and interrupting devices and more particularly to compact ground fault circuit breakers for residential applications.
Ground fault circuit breakers for alternating current distribution circuits are commonly used to protect people against dangerous shocks due to line-to-ground current flow through someone""s body. Ground fault circuit breakers must be able to detect current flow between line conductors and ground at current levels as little as 5 milliamperes, which is much below the overload current levels required to trip conventional circuit breakers. Upon detection of such a ground fault current, the contacts of the circuit breaker are opened to deenergize the circuit.
A differential current transformer, referred to as the ground fault or sense transformer, is normally used to sense these ground fault currents. The sense transformer has as its primary windings the conductors of the distribution circuit being protected, which are encircled by the core, and a multi-turn winding wound on the core. (In the case of a one pole breaker, the line and neutral conductors both go through the sense transformer core, and in the case of a two pole breaker, the two line conductors and the neutral conductor all go through this core. For the sake of example, the following discussion relates to a one pole breaker.) During normal conditions, the current flowing in one direction through the line conductor will return in the opposite direction through the neutral conductor. This produces a net current flow of zero through the transformer, and the multi-turn winding provides no output. However, if a fault (that is, a leakage path) is established between the line conductor and ground, return current will bypass the transformer and flow through the ground back to the grounded side of the source supplying the circuit. Thus, more current will be flowing in one direction through the transformer than in the other, producing a current imbalance. Such a current imbalance produces uncancelled flux in the sense transformer""s core, resulting in an output from the multi-turn winding that trips the circuit breaker mechanism.
A ground fault circuit breaker must also trip upon occurrence of an inadvertent short between the neutral conductor and ground that may occur due to a fault such as a wiring error by the electrician installing the circuit breaker. Such a leakage path on the load side of the sense transformer does not in itself produce a shock hazard; however, the occurrence of a grounded neutral at the same time as a ground fault on a line conductor will cause the ground fault circuit breaker to be less sensitive in detecting ground fault currents, thereby creating a hazardous situation. A neutral-to-ground fault reduces the sensitivity of the sense transformer as a ground fault sensing device because such a fault tends to provide a return current path via the neutral conductor for a large portion of the line-to-ground leakage current. To the extent that line-to-ground leakage current returns to the source via the neutral conductor, it escapes detection by the sense transformer. Consequently, the sense transformer may not respond to a hazardous ground fault.
An additional current transformer, referred to as the ground neutral transformer, is commonly used to detect neutral-to-ground faults. In one known application, the ground neutral transformer comprises a core that encircles the neutral conductor (the ground neutral core can, but need not, encircle the line conductor too) and has a multi-turn winding wound thereon. When a neutral-to-ground short or fault occurs, an inductively coupled path between the sense transformer and the ground neutral transformer is closed. The resultant coupling produces an output in the ground fault sense transformer that trips the circuit breaker mechanism.
Such circuit breakers provide generally satisfactory operation. However, because of the requirement for two current transformers, it can be difficult to package both transformers, together with the large #12 or #14 conductors and a printed circuit board (which contains standard circuit breaker circuitry), into the small allotted volume provided in existing circuit breaker housings. This is particularly the case in residential applications for which compact, half-inch circuit breakers are now available. In addition, because of the compactness of these circuit breakers, portions of the conductors are located very close to the transformer cores. It has been noted that when this occurs, the core material may saturate near the point of closest approach. If a region of the sense transformer core becomes saturated, a large asymmetry in its magnetic properties is introduced and this will degrade the sense transformer""s ability to detect ground fault currents. This problem is particularly acute when the transformer core is made from materials, such as ferrite, which have relatively low saturation magnetizations. Use of materials such as ferrite is preferred because they are less expensive than high saturation core materials such as those available under the trademark Permalloy.
Accordingly, there is a need for a compact ground fault circuit breaker arrangement that is resistant to saturation problems.
The above-mentioned need is met by the present invention which provides a circuit breaker for use with a circuit having one or more line conductors and a neutral conductor. The circuit breaker includes a first transformer having a first toroidal core and a second transformer having a second toroidal core. The first and second cores are arranged concentrically with one another on a printed circuit board, which is disposed with a compact circuit breaker housing. The line and neutral conductors pass through both transformer cores.
The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.